1. Field of the Invention
The present invention relates generally to the field of optoelectronic devices and, more particularly, to devices which function as extremely high speed optical shutters, and to applications of such devices.
2. Description of the Related Art
A number of important commercial, scientific, medical and military signal processing or sampling applications require high speed conversion of time-varying analog waveforms into digital form. Such higher digitization speed is useful because, among other reasons, it provides better spatial resolution for lidar and range-finding, better time resolution for clock synchronization protocols, better instrumental resolution for sampling oscilloscopes, and better channel separation for wideband receivers. Higher speed analog-to-digital (A/D) converters are additionally sought because there is often a threshold conversion rate below which the application requiring the samples is infeasible, such as digital receivers operating in a particular microwave band.
It is known that an analog serial-to-parallel (S/P) converter can be used to parcel out portions of a time-varying waveform to parallel AID converters working in parallel. In such systems, the quality of the S/P converter stage bounds the subsequent fidelity of the overall converter, so manufacturability and control of noise are crucial considerations. Unfortunately, today""s all-electronic S/P converters operate well below 60 gigasamples per second (GSPS), which is not much faster than today""s state-of-the-art electronic AID converters (e.g., 6 GSPS claimed by Rockwell in the laboratory, for which, to applicants"" knowledge, no publicly-available enabling disclosure exists), so there is little fan-out by the S/P converter. By way of comparison, all-optical switching/sampling phenomena can occur at intrinsically higher speeds than analogous electronic phenomena, since electron mobility in the solid state is up to 10,000-fold slower than the speed of light; the advantage is lessened by the ratio of device sizes.
Accordingly, it is among the objects of the instant invention to provide one or more of the following: (i) an improved A/D converter operating at optical speeds yet benefitting from progress in the speed of electronic devices (ii) an integrated, fast A/D converter; (iii) an improved A/D converter with calibrated outputs and servo-controlled input range; (iv) an improved A/D converter with a parallel output signal; (v) an improved A/D converter in a circuit; (vi) an A/D converter embedding a fast analog S/P converter (vii) an integrated fast analog S/P converter; (viii) a fast analog optical S/P converter employing a TOAD (as defined below); (ix) an improved, fast analog S/P converter with adjustable input dynamic range; (x) an improved S/P converter with physical clocking; and (xi) a device capable of converting an optical waveform into piecewise portions employing a TOAD.
In accordance with these and other objects, there is provided an analog-to-digital converter, which includes a first input port configured to receive an analog optical waveform. The converter also includes a splitter connected to the first input port and configured to split the analog optical waveform into a plurality of identical waveforms. The converter further includes a second input port configured to receive a clock signal having a predetermined clock period. The converter also includes a delay circuit configured to receive the clock signal and to output a plurality of delayed clock signals each having different a different delay with respect to others of the delayed clock signals. The converter still further includes a plurality on optical shutters configured to respectively receive the plurality of identical waveforms on an input port thereof, and configured to receive a corresponding one of the plurality of delayed clock signals on a control port thereof, each of the plurality of optical shutters having an output port for outputting the corresponding one of the identical waveforms within a time period in a range of 0.01 psec to 100 psec. The converter also includes a plurality of photodetectors respectively connected to the output ports of the plurality of optical shutters and configured to convert an input optical signal into an output electrical signal. The converter further includes a plurality of electrical analog-to-digital converters respectively connected to the output ports of the plurality of optical shutters and configured to perform an analog-to-digital conversion of the corresponding electrical signal into a digital signal. The number of electrical analog-to-digital converters in a fully populated configuration is such that a conversion time of said analog-to-digital converters divided by the number of electrical analog-to-digital converters is slightly less than the time period of the optical shutters.
Aspects of the system and method according to the invention exploit and improve 35 upon recent advances in high speed optical shutter technology, notably the Terahertz Optical Asymmetric Demultiplexer (TOAD), disclosed in U.S. Pat. No. 5,493,433, which is incorporated in its entirety herein by reference. Whereas most optical shutters require impractically long interaction lengths, high power, or both; the TOAD is compact and low power.
The TOAD exploits a strongly non-linear optical effect which allows a gating pulse to cause either 0 or pi radians of phase delay in a signal introduced into an interferometer. The phase delay switches off after a brief interval, of the order of 1 picosecond (psec), so a signal beam meeting with itself in a TOAD interferometer will emit only the waveform sampled by the 1 psec shutter window. In contrast to conventional semiconductor logic gates, the TOAD does not try to switch and reset in a small multiple of the shutter cycle time. Rather, the TOAD can undergo an Open/Off cycle only once before it needs to recover, typically for 100 psec. When combined with a precise optical delay line, each TOAD can be used to sample (read) or inject (write) a signal""s amplitude or intensity in the shutter window time. The TOAD is then latent, waiting for the electronics to invoke it again (e.g. every 4 nanoseconds (nsec) for 250 MHZ CMOS).
Utilizing such high speed optical shutters, in accordance with the system and method according to the invention, it is possible to provide high speed, high quality S/P conversion, and therefore high speed A/D converter systems embedding a fast analog optical S/P converter with fan-out to whatever degree is necessary to support operation beyond 1000 gigasamples per second (GSPS). Also, in accordance with the invention, systems may include all-electrical A/D converter devices, thus providing a back-end with optimal price/performance. Preferably, the balance between more fan-out in the S/P converter and more, lower-cost slow electrical AID converters in the back-end is optimally selected, based upon overall price, performance, manufacturability, or other criteria. While the present invention may be, and preferably is, practiced using the TOAD, the concepts, teachings, and applications described herein below are by no means limited to TOAD-based systems, and will work with other optical shutters, other sequences of functional units, and with novel electrical A/D converters.
In accordance with the invention, the TOADs will preferably be solid state and formed by mass production processes. For instance, integrated semiconductor optical amplifiers (SOAs under development at Princeton University, British Telcom, NEC, and elsewhere can be combined with integrated optical paths to form complete TOAD devices. SOAs can advantageously be formed using materials from columns III and V in the periodic table (e.g. GaAs), or II-VI materials; a plurality of optical paths can be formed on a low cost substrate; and the two monolithic constructions bonded together. Additionally or alternatively, a plurality of optical paths may be formed on the same substrate as the TOADs by additive, subtractive, or even selective phase-change direct-write processes. One may apply light, charged particle beam, heat, or other known methods to cure the waveguide material or optical interconnects.
In accordance with a preferred embodiment of the instant invention, the time-varying magnitude of an input signal is modulated by amplitude or intensity. In more general embodiments, modulation of the magnitude comprises a combination of amplitude modulation, polarization modulation, phase modulation, and frequency modulation. These can all be exploited in their own right by appropriate TOAD embodiments, or reduced to amplitude or intensity modulation and treated as in the preferred embodiment. For instance, a polarized signal can be converted into an intensity modulated signal by being passed through a cross-polarizer. A frequency modulated signal can be converted into an amplitude modulated signal in a number of ways, including filtering with a color-sensitive transmitter or reflector; or diffracting or refracting and then using position, time of flight, or phase information. A frequency modulated signal can be converted into an intensity modulated signal by interfering it with a coherent reference beam of comparable intensity and sampling the beat frequency. In general, modulation can be permitted to occur within the TOAD, instead of just prior to it. For instance, a phase modulated signal can be converted into an intensity modulated signal by being superposed on a coherent reference signal of comparable intensity within the TOAD interferometer itself, thus replacing the input splitter described in the ""433 patent with a bypass on one side and a reference beam for differential measurement on the other side. This brief description is exemplary and not comprehensive. As those skilled in the art will appreciate, other methods are known, and it is anticipated that there will be future embodiments of optical shutters which will be appropriate for use in this invention.